1. Field of the Invention
This invention relates to appliance protection circuits that prevent electrical shock hazards arising from a hot to ground fault or a hot to neutral fault, and that further regulate temperature in the load.
2. Description of Prior Art
Any electrical device ("the load") requires the flow of electrical current in order to operate. An analogy is the flow of water through an aquarium filter. A pump takes in water from the aquarium and increases the pressure (analogous to an increase in electrical voltage) to force the water through a tube (the tube is analogous to the electrical conductor or wire) to the filter (analogous to the electrical load). The flow of water current through the tube is analogous to the flow of electrical current in a wire. Most of the water pressure is "used up" in passing through the filter so that the water coming out of the filter has a relatively low pressure. A hose conveys the low pressure water from the filter outlet back to the aquarium. If there is a hole in the tube connecting the pump and the filter then some of the water will go through this hole to pass from the high pressure in the tube to a lower pressure outside the tube. This constitutes a water leak.
In somewhat the same way, an electrical device or load receives electrical energy from one terminal of an electrical outlet or source (the so-called high voltage or "hot" side), electrical current flows to the device through an electrical conductor or wire (the hot conductor), this current passes through the load and is then returned to another terminal of the electrical outlet through another wire called the neutral wire. The neutral wire will have a very low voltage (electrical pressure) because most of the voltage will be "used up" in the act of forcing electrical current through the load.
The two wires that connect source and load may have a coating of rubber or some other electrical insulating material or they may be bare, in which case air, which is a good insulator, functions to inhibit electrical current flow outside of the wire. Since the human body presents a relatively low resistance to the flow of electrical current, if a person comes into contact with a high voltage object such as the hot conductor, while also making contact with a lower voltage object, then an electrical leakage current that is proportional to the voltage difference will flow through the person and may cause injury or death. If the lower voltage object that the person comes in contact with is electrically connected to the earth (ground) then this is called a hot to ground fault. If the lower voltage object that the person comes into contact with is the neutral wire, this is called a hot to neutral fault. If the person makes contact between the neutral wire and ground this is called a neutral to ground fault. Since the voltage difference between neutral and ground is generally small (because the neutral line is connected to ground at a breaker box), a neutral to ground fault is generally considered to be less hazardous than a hot to ground or a hot to neutral fault.
Electrical current is the flow of electrons. Since electrons are not created or destroyed, any functioning electrical appliance will require both an entry path and an exit path in order for electrical current to flow. In an electrical appliance, electrons may exclusively enter on one path and exit on a second (direct current or DC operation). For most household appliances that operate from a plug, electrons will sometimes enter path one and exit path two and sometimes enter path two and exit path one. This is known as alternating current or AC operation. A special case of AC operation is the so-called half wave AC (also known as pulsating DC) whereby over regularly repeating periods of time (or cycles), electrical current will flow in one direction for a portion of the cycle and will be blocked from flowing for another portion of the cycle. Although the direction of current flow in half wave AC operation is unidirectional, it is considered to be alternating current since the magnitude of the electrical current varies in a cyclical fashion.
A common source of electrical injuries in the home occurs when people place radios or similar electrical devices near their pool or bath tub while swimming or bathing. If the radio is knocked into the water, it can create electrical leakage current through the water to ground, in other words, a hot to ground fault. A hot to ground fault can also occur when a person touches an electrically hot conductor while standing on or touching a grounded conductive surface. When sufficient current passes through a person, electrical burns or electrocution may result. Many electrical appliances such as heaters, hair dryers, electric razors and pumps are used near water and present this type of hazard.
An additional hazard for the user of an electrical appliance occurs when the user comes into contact with both a hot and a neutral wire, establishing a path through the user's body for electrical current to flow. This is known as a hot to neutral fault and can lead to electrical burns and electrocution. A hot to neutral fault can occur when the cord on an appliance is broken or the insulation is frayed or otherwise damaged. A hot to neutral fault can also occur when a power hand tool or electrical lawn appliance comes into contact with and cuts the power cord.
The U.S. Consumer Product Safety Commission compiles annual estimates of injuries in the U.S. due to consumer products. For the year 1993, this organization estimates that there were over 8200 injuries due to electrical shock and over 8900 additional injuries due to electrical burns. The products in which these injuries occur are grouped in such diverse categories as extension cords (an estimated 129 shock and 481 electric burn injuries), power drills (an estimated 17 shock injuries) aquarium accessories (an estimated 112 shock injuries), Christmas tree lights (an estimated 69 shock and 34 electrical burn injuries) and hair dryers (an estimated 103 shock and 491 electrical burn injuries).
Protection circuits for hot to ground faults, commonly known as ground fault interrupters or GFI's, are presently required by code for the bathrooms of most new homes and commercial buildings. Such circuits are also required for Underwriter's Laboratories approval in hair dryers and are built into the plug of U.L. approved hair dryers. Of the GFI circuits presently available on the market, all use a current imbalance in a current sense transformer as the means of detecting a fault. Such circuits are described, for example, in U.S. Pat. Nos. 4,216,515 (Van Zeeland), 4,353,103 (Whitlow), 4,979,070 (Bodkin) and 5,200,873 (Glennon). None of these inventions will protect against a hot to neutral fault in the appliance wiring.
U.S. Pat. No. 3,997,818 (Bodkin) discloses a GFI circuit in which a silicon controlled rectifier (thyrister) is used for half wave loads, using a resistor/capacitor combination to implement phase control. During the 0 to 180 degree portion of the cycle, load current is blocked by an unfired thyrister and the gate controlling capacitor is in a discharged condition. If a fault is present during the first 180 degrees, the gate controlling capacitor is reversed charged through the fault resistance and will inhibit the triggering of the thyrister during the next half cycle. The problem with this approach is that in the absence of a fault it takes a significant amount of time to charge the capacitor sufficiently to fire the thyrister during the 180 to 360 degree interval and as a result, the load receives electrical current significantly less than 180 degrees (half) of each cycle. This invention (U.S. Pat. No. 3,997,818) is also said to work for full wave loads by using two of the half wave circuits, each tied to the same neutral connection. In the full wave implementation a three wire power cord is thus required between plug and the load with two of the wires being a hot and the third being a neutral.
A device called an immersion detection circuit interrupter (IDCI) detects the occurrence of electrical leakage due to water immersion and opens a circuit breaker, effectively protecting appliances such as hair dryers from hot to ground faults due to, for example, dropping a hair dryer into a bathtub. Such immersion protection devices are described in U.S. Pat. Nos. 4,159,517 (Bodkin) 4,797,772 (Kaplanis) and 5,184,271 (Doyle and Rivera). These immersion detection circuits require a three wire cord in order to protect against a hot to ground fault. In addition, they will not afford protection against a hot to neutral fault.